When time-sensitive data, such as voice or video data, is sent between a source node (Master) and destination node (Slave) over a packet network, the source and destination nodes will have local clocks that are not synchronized. Some method is needed for recovering the source clock at the destination node in order to recover the time-sensitive data.
Existing time distribution systems, such as Network Time Protocol (NTP), IEEE-1588, that use packet networks to synchronize clocks send so-called timing packets in both directions between the source and destination nodes. From these packets the transmission time is compared with the arrival time to enable the precise packet transit time to be determined. The clock frequency of the destination node is adjusted to retain a measured constant transit time for all timing packets passing through the network. In this way, the clock at the slaves can be synchronized to the clock at the master. This method is referred as ‘Constant-Transit-Time’ (CTT) clock distribution.
A CTT-method can combine the transit time for packets for both directions between source and destination node. On the assumption that both paths are subject to the same delay, the steady transit time can be eliminated so the slave clock runs with no offset relative to the Master node. The technique is referred to as ‘Same-Time’ synchronization.
Existing clock synchronization methods such as the Network Time Protocol (NTP) or Precision Time Protocol (PTP) from IEEE-1588, are based on the fact that packets travel with a Constant Transit Time (CTT-method) over the network. Such methods are sensitive to packet delay variations.
The CTT method loses precision when timing packets are queued at switching components and when the switching components show high timing granularity, which means that the packets are sent out at discrete time intervals. In Ethernet networks a granularity has been measured that relates to the duration required to send 3, 64 or 72 bytes over a 100 Mbps link. TDM based networks, such as T1 and E1, show a time granularity related to the frame rate of 8 kHz.
It is not possible to determine the transit time to a degree of precision better than the underlying timing granularity, and so the granularity of the network results in a loss of precision in the timing information exchanged between the source and destination nodes.